This invention relates to a surface mountable light emitting device, particularly, a surface mountable light emitting device having a high power capacity.
Conventional light emitting diodes provide a light emitting semiconductor chip within a metal cup, a lead wire to a further contact on the chip, a bullet lens over the structure and a body around the structure. Often the body would be formed integrally with the bullet lens. Both the cup and the lead wire are attached to legs extending from an underside of the body for connection through a printed circuit board into a suitable circuit.
The manufacture of products that utilize large numbers of light emitting diodes may favor the use of surface mountable devices. The attachment of many such LEDs to, for example, a printed circuit board holding the driving circuitry can be achieved considerably more economically by automated machines. If such a machine can operate on a single side of the printed circuit board to place and secure the LED, significant savings may be made, and the reverse side of the printed circuit board can be left free for the provision of the driving circuitry. All of this requires a surface mountable device that avoids the traditional placement of the legs of the LED through a printed circuit board and soldering on the reverse side of the board.
A variety of methods have been attempted to achieve a suitable surface mountable light emitting device. Usually, such methods have involved the protrusion of the lead wire and a connection to the lead frame at the side of the device for attachment to the surface on which it is to be mounted. Although surface mountable, such connections are arranged around a perimeter of the device, which limits the density at which they may be mounted on the surface.
A further problem with light emitting devices occurs more permanently with high power devices. An LED running at high power, such as at one watt generates a significant amount of heat. This heat can deteriorate the performance of the LED or, over time, lead to the destruction or burn out of the LED.
Although the heat may be dissipated by the surrounding apparatus, this still requires the transfer of the heat from the source, to outside of the LED. The legs extending from the body of the LED provide a relatively small thermal pathway, and do not allow sufficient heat dissipation to allow high power units on the order of one watt.
A yet further difficulty in the subject art arises in the manufacture of LEDs. It is difficult to provide a process that allows easy manufacture of LEDs with a minimum of components while assuring the requirements, e.g., greater heat dissipation, of high power units are met.
Also, conventional LEDs are optically unsuitable for direct installation into devices such as headlamps or flashlights that use parabolic reflectors. This is because the bullet lenses used form a narrow beam that completely misses a nearby parabolic reflecting surface. Using, instead, a hemispherically emitting non-directional dome, centered on a luminous LED die, gives a maximum spread commercially available, a Lambertian pattern. Since xcex8 for a typical parabolic flashlight reflector extends from 45xc2x0 to 135xc2x0, an LED with a hemispheric pattern is still mismatched with respect to a parabolic reflector because the LED""s emission falls to zero at only xcex8=90xc2x0. This results in a beam that is brightest on the outside edges and completely dark halfway in to its center. Worse yet, even this inferior beam pattern from a hemispheric LED requires that the LED be held up at the parabola""s focal point, several millimeters above the socket wherein a conventional incandescent bulb would be installed.
There is thus a need in the art for an effective and optically suitable surface mountable light emitting device (LED) that avoids the traditional placement of the legs of the LED through a printed circuit board and soldering on the reverse side of the printed circuit board, provides sufficient heat dissipation, allows easy manufacture with minimum components, ensures the requirements of high power usage are met, and is optically suitable for direct installation into devices that use parabolic reflectors as replacements for tungsten filament light bulbs.
The present invention advantageously addresses the needs above as well as other needs by providing a surface mountable light emitting device that avoids the traditional placement of legs of the LED through a printed circuit board, and soldering of the legs to the printed circuit board on the reverse side of the printed circuit board, provides sufficient heat dissipation, allows easy manufacture with minimum components, ensures the requirements of high power usage are met, and is optically suitable for direct installation into devices that use parabolic reflectors and replacement of tungsten filament light bulbs.
In one embodiment, the invention can be characterized as a high power, surface mountable light emitting device comprising a light emitting semiconductor chip, a thermally and electrically conductive lead frame connected to said chip and exposed over a substantial portion of the underside of the device, a lead wire from said chip to a contact exposed at least partially on a side of said device and a lens over said chip.
The lens comprises a lower transfer section and an upper ejector section situated upon the lower transfer section. The lower transfer section is operable for placement upon the light emitting semiconductor chip and operable to transfer the radiant emission to said upper ejector section. The upper ejector section is shaped such that the emission is redistributed externally into a substantial solid angle.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings, which set forth an illustrative embodiment in which the principles of the invention are utilized.